In Magnetic Resonance Imaging (MRI), a gradient amplifier is typically used to provide current for three magnetic field gradient coils to provide 3-dimensional spatial encoding of atomic spins located in a magnetic field.
These gradient amplifiers are typically characterized by high peak power (several 100 kW up to 2 MW for present-day specimens) and high precision of the generated current waveforms. Circuits consisting of series-connected full bridges using pulse-width modulation (PWM) have been used to construct gradient amplifiers.
This circuit topology is known under several names, such as “stacked H-bridges”, “cascaded H-bridges”, or “cascaded multicell converter”. The state of the art gradient amplifiers are switch-mode amplifiers, consisting of a series of H-bridges with solid state switches.
A fundamental circuit in power electronics is the canonical switching cell. The canonical switching cell is typically discussed using ideal switches. However a more practical implementation is using Insulated Gate Bipolar Transistors (IGBT) with anti-parallel diodes as switches.
The canonical switching cell is used to control the power flow and thereby the exchange of energy between two systems. Two switches are operated such that the load is connected to either the positive or negative terminal of a voltage source. The switches are operated in a manner such that exactly one of these is closed at any time. Closing both switches is prohibited as this would create a short circuit across the voltage source and thereby possibly cause unlimited current flow; opening both switches would obstruct the current from the current source on the right to flow, possibly causing unlimited voltage rise. Two trigger signals control the state of the two switches such that when a trigger signal is 1 the switched is closed, and when the trigger signal equals 0 then the switch is open. Due to the constraint discussed above the two trigger signals are logical inverses of each other. Note that this is a very general and conceptual circuit: depending on the polarity of the voltage V and of the current I the power flow can be in either direction.
The combination of two IGBT switches is defined as a phase leg; the origin of this name being that three of these circuits are necessary to build a three-phase voltage source inverter, which is presently the circuit of preference to drive medium power (ca. 100 W to 1 MW) induction motors.
The most common way a single phase leg is used is to control the power flow between the two attached systems is by using Pulse-Width Modulation (PWM). The simplest example of PWM is where two gate signals show a repetitive pattern in time. The first gate signal is turned on and conducting during an interval δTk, and the second gate signal is turned on during the complementary interval (1−δ)Tk, where Tk denotes the repetition interval. The time interval δTk can also be expressed as a percentage of the time that a gate is turned on for one period of the PWM cycle.
For magnetic resonance imaging systems, H-bridges are switched at a fixed frequency of e.g. 20 kHz between a first and a second switching state. The time spent in each of the two switching states determines the time-averaged output voltage. The IGBT's are partly switching losses (e.g. 50%) and partly conducting losses (also 50%). The IGBT's are switched continuously. A minimum value of the switching frequency is needed to achieve a certain bandwidth.
In J. Sabatze et. al., “High-Power High-Fidelity Switching Amplifier Driving Gradient Coils for MRI Systems,” 35th Annual IEEE Power Electronics Specialists Conference, 2004, pages 261-266 discloses a method of controlling the H-bridges in a gradient coil power supply to reduce switching losses. In this paper, two high voltage bridges supplied with 800V are not pulse modulated and only provide voltage when more than 400 V are needed. When not in use they remain in a freewheeling model with no output voltage.